Competition between stable and metastable eutectic growth in Sn-Ni alloys

Abstract Metastable Sn-NiSn 4 eutectic can form in electronic solder joints and is a reliability concern. Here the competition between stable Sn-Ni 3 Sn 4 and metastable Sn-NiSn 4 eutectic growth is studied during unidirectional solidification. The stable and metastable eutectic points are measured, the dynamics of eutectic growth and the transition between the two eutectics is investigated by synchrotron radiography, and the crystallography of eutectic growth is measured by EBSD. It is shown that the Sn-Ni 3 Sn 4 eutectic has a highly irregular morphology with diverging and converging rods because monoclinic Ni 3 Sn 4 grows only along [010] without a reproducible orientation relationship (OR) with Sn. The metastable Sn-NiSn 4 eutectic has a more regular broken-lamellar morphology and a reproducible OR with Sn. The critical velocity between the two eutectics is less than 1 μm/s which is discussed in terms of the small temperature difference between the two eutectic points and the kinetic growth advantages of the metastable eutectic.